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| Home > Publications database > Chemische Reaktionen suprathermischer Wasserstoffatome in der Koma von Kometen |
| Book/Report | FZJ-2018-03357 |
1990
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/18821
Report No.: Juel-2409
Abstract: Photodissociation of H$_{2}$O molecules in cometary comae creates suprathermal hydrogen atoms with kinetic energies in the eV range. Other sources for hot hydrogens, such as collisions of solar wind and pick-up ions with gaseous coma constituents or the charge neutralization of these ions contribute only a few percent. The preferential reaction of H atoms with H$_{2}$O, OH and CH$_{4}$ is hydrogen abstraction to form HO, O, CH$_{3}$ and H$_{2}$. Energy dependent cross sections for the hot reactions are obtained via an inverse Laplace transformation from thermal reaction rates. This new approach to non-equilibrium kinetics is valid in the region of the reaction threshold energies. It enables to calculate reaction rates for hot atoms with eV energies. Spatial and energy distribrution of the hot hydrogens in the coma are calculated by two models: a) the vector model of Festou, based on a linear expansion of hydrogen, and b) a Monte-Carlo simulation whichaccounts for changes in direction and velocity due to collisions. The results obtained via the two independent approaches on the basis of data for P/Halley differ by only 20%. For further calculations, the vector model lends itself to describe the distribution of energetic atoms in the coma in a simple and effective way. A density maximum of hot H atoms is observed at distances from the nucleus of 100 to about 1000 km. In this region the H-abstraction for H$_{2}$O amounts to about 15%, for CH$_{4}$ to 40%, and for OH to max. 60% of the respective photodissociation rate. In particular, the production of CH$_{3}$ radical by H-abstraction is by a factor of 3 higher than that by photodissociation. Suprathermal reactions, thus, provide a significant contribution to the destruction of molecules in the inner coma where the gas density is high. In particular the destruction of H$_{2}$O by hot hydrogen is higher by a factor of 1.5 than that by all other thermal ion molecule reactions. This demands consideration of suprathermal reactions when modelling coma chemistry.
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